This invention relates to bottom structures for containers, and more particularly, to improved bottom structures for plastic bottles of the type suitable for containing effervescent or carbonated beverages.
The bottling of carbonated beverages in plastic presents a number of problems, many of which arise in connection with the base or bottom structure of the bottle. Mere duplication in plastic of traditional glass bottom configurations is unsatisfactory because of the tendency of plastics to creep or become distorted under pressure, especially in the presence of the elevated temperatures which may be encountered during shipment and storage. Such distortion may alter the shape and dimensions of traditional bottom configurations to the extent that the level of liquid within the bottom falls below the fill line, thereby threatening customer acceptance or satisfaction, and the bottle may become a so-called rocker; that is, it may become unstable on a horizontal surface, with the probability of similar adverse reaction on the part of the customer.
On the other hand, it is frequently desirable that the inner and outer shapes and dimensions of plastic bottles approximate those of glass bottles of the same capacity so that they may be handled by existing equipment and, in certain instances, assist customer identification of the particular product they contain. In any event, they should be aesthetically attractive.
A plastic bottle, when filled with a carbonated beverage and capped, must be able to withstand the impact of falling from at least a moderate height onto a hard surface, and the precipitous rise in internal pressure which accompanies the impact. While this requirement also affects selection of materials and bottleforming techniques, it is an extremely important consideration in the contemplation of bottom design.
Finally, the optimum bottom structure is one which not only meets the foregoing criteria but which may be readily formed with an economy of material, without unduly expensive or elaborate equipment, and without intricate or additional manufacturing steps.
The copending U.S. patent application Ser. No. 335,974, now U.S. Pat. No. 3,871,541 filed Feb. 26, 1973 in the name of D. Adomaitis and assigned to the assignee of the present invention, discloses a container in which the outer surface of the bottom structure comprises, briefly, a central concave dome portion, a convex annular rim portion circumscribing the dome portion and merging therewith and with an adjacent cylindrical portion of the side wall of the container, and a radial array of convex foot portions extending axially outwardly of the rim portion. Each of the foot portions merges at its radially inner end with the dome portion, at its radially outer end with the side wall, and at its lateral margins with the rim portion to form a shallow rib portion between adjacent foot portions.
Noting that plastics are weakest in tension, such a construction exposes the dome portion to compressive stresses only, and among other advantages, arrests the tensile and flexural stresses at the base of the dome portion while permitting an economy of material in forming the bottom structure.
In the prior application, the dome portion is shown to be a segment of a sphere (the sphere being commonly recognized as the optimum pressure-bearing surface), whereby the radial profile of the outer surface of the dome portion comprises a concave arc having its center of curvature on the central axis of the bottle. The radial profile of the outer surface of each foot portion comprises a single convex outer arc tangential to the inner arc of the dome portion and to the cylindrical side wall portion. The axially outermost point of the latter arc comprises a support point for the bottle when it is at rest on a horizontal surface in an upright position. As is readily apparent, in order effectively to distribute and dissipate the forces arising from internal pressures and/or impact with a hard surface, curved surfaces are used virtually throughout the bottom structure, and to avoid undue stress concentrations, adjacent surface portions are smoothly merged or blended with each other.
It has been found that the stresses arising in such a bottom structure may be reduced by increasing the radius of curvature of the arc of the foot portion. However, as this radius is increased, the support point is moved inwardly toward the central axis of the bottle, and the bottle therefore tends to become less and less stable when supported on a horizontal surface. Upright stability is especially critical in the case of certain types of bottle conveying equipment presently in use.
If, on the other hand, the radius of the arc of the foot portion is made smaller to enhance stability, and the radius of the arc of the dome portion thereby increased, the material of the dome portion must be made thicker to avoid snap buckling; that is, the sudden eversion of the dome portion under pressure. A further problem arises in that the radially outer end of the foot portion becomes an increasingly sharp corner as the radius of the outer arc decreases in length. In blow-molding it becomes more and more difficult to fill the corresponding corner of the mold. Still further, capacity is reduced and more material is required, or still more material must be used to achieve similar capacity.